Interferometers have been known and used for a long time. Interferometry is a widely used method for measuring surface profiles (often to nano-meter resolutions) and other physical properties of materials, gases and liquids. There are many types of interferometers, characterized by their optical designs and layouts. Some classical types are Twyman-Green, Fizeau, Michaelson, Mach-Zender, and Fabry-Perot. Each of these interferometer types produces interference patterns, called interferograms. These interferograms (see FIG. 6) can be used to analyze characteristics of an object under test.
Interferograms are generated by the interference of a test wavefront and a reference wavefront. The test and reference wavefronts typically originate from a common “source” wavefront; they are obtained by splitting the source wavefront. The test wavefront then obtains its “test” information by interacting with the object under test (typically by reflecting off of, or transmitting through a test object). Similarly, the reference wavefront obtains its “reference” information by interacting with a “known” reference object, such as a super polished flat glass plate. Superimposing or interfering these two wavefronts spatially (i.e. on a flat screen, or on an image sensor such as a CCD) produces an interferogram.
Interferometers require coherent superposition of a “test beam” (of light) with a “reference beam” resulting in the formation of an interferogram in the overlapping region of the two beams. This interferogram data can then be captured using various types of detectors, such as a camera, for analysis.
The spatial distribution of intensity levels within the interference pattern (see FIG. 6) relates to differences in the phase of the test and reference wavefronts. Note that the reference wavefront is acted on by a known “measurement standard,” such as an optical “reference” surface, and the test wavefront is acted on by the unknown object under test. Measuring the difference between the two wavefronts allows the test wavefront to be determined. In other words, the process is akin to comparing the “unknown” test wavefront to a “known” standard, the reference wavefront.
A single interferogram is usually insufficient to obtain the accuracy required for many applications. A variety of methods have been developed to acquire multiple phase-shifted interferograms as a means to increase accuracy and resolution of the measurement. Phase-shifting techniques require altering the phase between the two interfering wavefronts by introducing controlled phase delays between the test and reference beams. These added phase-shifts supply additional information that can be used to compute the test wavefront significantly more accurately. Almost all current techniques of phase shifting use sequential or “temporal” methods to introduce phase differences while multiple interferograms are acquired serially in time. However, in practice, these temporal methods cannot be used effectively in the presence of relatively fast changing environmental conditions (such as vibrations, air turbulences, etc), or when the object under test cannot be stabilized (i.e. vibrating), or when the object under test is in motion. For example, problems can arise because the interferometer typically acquires three to twelve frames (images) or phase shifted interferograms, serially in time, (typically spaced 30 ms apart for standard video), and during this acquisition time (for three to twelve frames), any vibration that occurs between the test and reference object often causes measurement errors.
Thus, methods have been developed to acquire multiple phase-shifted interferograms simultaneously. These methods usually require that the reference and test beams (“beams” and “wavefronts” used interchangeably herein, with a “wavefront” being understood as propagating along the optical axis and sweeping out a volume that defines the light beam) be orthogonally polarized, thus allowing independent access to either one of these beams (such as via polarization optics), even when they are spatially overlapped. With this “independent access”, multiple phase-shifts can be introduced simultaneously (as opposed to temporally), by retarding or advancing the phase of one beam with respect to the other. Altering the phase of a beam is typically accomplished through the use of wave plates or polarization beam splitters. In practice, this is accomplished by splitting the superimposed test and reference beams into three or more channels with each new channel having orthogonally polarized test and reference beam components. For each new channel, one of the beam components (test or reference) is then phase-shifted relative to the other beam component. This phase shift, or phase delay, is adjusted to be different in each channel. There are multiple methods for splitting the superimposed test and reference beams into multiple channels and multiple methods for phase-shifting within each channel. However, as discussed below in further detail, such known methods have disadvantages, including dissimilar beam paths attributable to different path distances and/or different optical elements.
U.S. Pat. No. 4,583,855 (Bareket) describes a Twyman-Green type interferometer with an arrangement for producing three simultaneous interferograms that are mutually phase shifted by 90° with respect to each other. This system relates to the use of test and reference beams that are mutually orthogonally polarized; however, the system is substantially asymmetric. As such, the system can be difficult to align and a constant magnification in all three beam paths difficult to maintain.
A system similar to Bareket's has been described by Sivakumar, (Optical Engineering, Vol. 42 page 367), which therefore shares most of the characteristics and issues of Baraket's system. This system is also directed to fixed phase shifts between each channel.
In the publication “Simultaneous Phase Shift Interferometer” (Proc. of SPIE, Advanced Optical Manufacturing and Testing II, January 1992) by C. Koliopoulos, a polarization type Twyman-Green interferometer is described. This system is also asymmetric and has different optical path lengths. This system may also suffer from complex geometry and being a relatively large size.
A publication entitled “Real-time fringe pattern processing and its applications” (Proc. of SPIE, Vol. 2544, pp. 74-86, 1995) by S. Nakadate describes a system of linearly cascaded, non-polarizing beamsplitters and a reflector. This system of beamsplitters is substantially asymmetric and non-standard. In a real world application, where imaging optics are present to image the object onto the cameras, the asymmetry of the optical paths in this system can make it difficult to provide in-focus positions of the imaging cameras and/or to maintain constant magnification. Other issues can also be raised due to the number and complexity of optical elements traversed in each of the beam paths.
Another method proposed by J. W. Schwider (German Patent No. DE 196,52,113,A1) uses a diffractive optical element to split a beam composed of mutually orthogonally polarized test and reference beams. Although diffractive elements appear attractive as beam splitters, actual diffractive components can be difficult and expensive to produce. This approach also involves a CCD camera whose resolution can affect the quality of the interferograms. A similar approach has been offered by Millerd, U.S. Pat. No. 6,304,330 and U.S. Pat. No. 6,552,808. Along with camera resolution issues, this method shares other drawbacks of the Schwider method, including sub-pixel alignment issues, and fixed at 90° phase shifts.
In view of the foregoing, major disadvantages of prior simultaneous phase-shifting methods include unequal path-lengths and dissimilar optical elements in each of the phase-shifted beam paths. This causes a variety of problems including different magnifications, unequal optical and polarization aberrations, and non-uniform light distributions in each of the phase-shifted paths.
Thus, there is a desire for an improved interferometer, in particular, a back end assembly adapted for use with an interferometer to render the interferometer less sensitive to vibrations or other disturbances so as to increase and improve performance capabilities. The desire includes greater and better performance capabilities in capturing data from stationary objects and from moving objects.